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Small Innovative Missions for Planetary Exploration

CubeSat Particle Aggregation and Collision Experiment

Project Introduction

We propose to build and fly the CubeSat Particle Aggregation and Collision Experiment (Q-PACE) to study low-velocity collisions between cm-scale and smaller particles in the protoplanetary disk. Q-PACE is a planetary mission to the orbital free-fall environment necessary for long duration microgravity experiments to study collisions in the early protoplanetary disk. For 40 years there have been two main ideas about how the pieces of future planets grow through the critical range from mm-sized aggregates and chondrules to km-sized planetesimals. These are gravitational instability (Goldreich and Ward 1973, Youdin and Chiang 2004) and growth by binary sticking collisions (Johansen et al. 2014). The experiments we propose will make it possible to determine whether collisional growth can proceed into this size range, confronting the decades-old question of how bodies grow past the meter-size barrier into planetesimals that can go on to become planets through gravitational accretion. Knowing the outcomes of collisions across a broad range of speeds and impactor properties will also enable us to connect the growth of the solid bodies to the properties of the particles observed in protoplanetary disks and meteorites. If dust agglomerates easily stick, the primordial grains are incorporated into bigger bodies in a fraction of the disk's lifetime. With the dust removed, the system's infrared excess is reduced (Dullemond and Dominik 2005). However, large infrared excesses are observed for many young stars with the Spitzer Space Telescope. Spitzer, Hubble and ground-based instruments all show that μm-sized dust is suspended in protostellar disks' atmospheres (Furlan et al. 2006, Watson et al. 2007, Pinte et al. 2008, Sargent et al. 2009), while the Atacama Large Millimeter Array senses mm-sized grains deep inside the disks (Peck and Beasley 2008, Dutrey et al. 2014, Testi et al. 2014). To understand what these observations can tell us about planet formation within the disks, we must know how the solid particles evolve in collisions. Q-PACE is a 2U CubeSat with a collision test cell and several particle reservoirs that contain meteoritic chondrules, dust particles, dust aggregates, and larger spherical monomers. Particles will be introduced into the test cell for a series of separate experimental runs. The test cell will be mechanically agitated to induce collisions, which will be recorded by on-board video for later downlink and analysis. Q-PACE is directly relevant to a number of NASA objectives and goals as stated in the 2014 NASA Strategic Plan, including Objective 1.5: "Ascertain the content, origin, and evolution of the solar system and the potential for life elsewhere" and Objective 1.6: "Discover how the universe works, explore how it began and evolved, and search for life on planets around other stars" by advancing our understanding of planet formation in our own solar system and around other stars. The objectives of Q-PACE require a long-duration and high-quality (low residual acceleration) microgravity environment that is only available on an orbital platform. The experiments proposed here will extend our understanding of the outcomes of relevant collisions between so-called "pebbles" (cm-scale objects) in the protoplanetary disk and the accretion of pebbles from smaller particles. The large number of collisions we will observe will enable a stochastic approach to the study of the evolution of particle size distributions in protoplanetary disks. Q-PACE results will be an advance in our understanding of the early-stage collisional evolution in protoplanetary disks. It extends the parameter range of collisions to smaller velocities than have previously been studied and expands the number of collisions observed by orders of magnitude over the current experimental database.
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